Shear Stress Measurement Technique Research Articles

Evaluation of mucoadhesive property and the effect of Boswellia carteri gum on intranasal vaccination against small ruminant morbillivirus infection (PPR)

ABSTRACTA study was conducted to evaluate mucoadhesive property and immunomodulatory effect of phytogenic gums from Boswellia frereana, Boswellia carteri andCommiphora myrrha on intranasal Peste des petits ruminants (PPR) vaccination in goats and sheep in an ex-vivo and in-vivo situations. Plant gums were purified, dried and compressed into 500gm tablets. Modified shear stress measurement technique was used on freshly excised trachea and intestine tissues of goat to measure peak adhesion time. Forty eight animals (24 goats and 24 sheep) were divided into eight groups (of 3 goats and 3 sheep) and immunized intranasally with gum-vaccine combinations in two ratios (1:1, 1:2). Antibody against PPR virus was measured on day 14, 28, 42 and 56 post vaccination using H-based PPR bELISA. The peak adhesion time of the different gums was transient. PPR virus antibodies were detected in all immunized goats and sheep but not in unvaccinated control. The best percentage inhibition was recorded for Boswellia carteri-vaccine combination group at a ratio of 1:1. Administration of Boswellia carteri-PPR vaccine combination through intranasal or subcutaneous route, elicited similar antibody titre, implying that the intranasal route may be used as a non-invasive alternative delivery in PPR vaccination of small ruminants.

A review of factors affecting the efficiency of clean-in-place procedures in closed processing systems

This paper reviews the current state of researches on improvement of Clean-In-Place (CIP) procedures in closed processing system thus saving energy, with a special attention paid to the hydrodynamic effects of cleaning fluid and the numerical and experimental approaches to investigate the identified controlling factors. The paper discussed the fouling problems of processing plants and the importance of sufficient CIP procedures, the forces contributing to cleaning with a special focus on the hydrodynamic effects. In general, it is possible to enhance hydrodynamic removal forces by local introduction of, among others, high wall shear stress and fluctuation rate of wall shear stress without consuming more energy. A theoretical model of particle removal in flow was also reviewed which supports the factors identified. The paper therefore further reviewed and compared the current state of modelling and experimental techniques on CIP improvement. To simulation the CIP process, it is necessary to consider 3D time-resolved Large Eddy Simulation with a Hybrid RANS-LES WMLES as Sub-Grid-Scale model because it captures both the mean and fluctuation rate of flow variables, while affordable for industrial flows. The wall shear stress measurement techniques and cleanablity test methods were also discussed and suggested.

An improved wall shear stress measurement technique using sandwiched hot-film sensors

ABSTRACT In this letter we present a novel wall shear stress measurement technique for a turbulent boundary layer using sandwiched hot-film sensors. Under certain conditions, satisfactory results can be obtained using only the heat generated by one of the hot-film and a calibration of the sensors is not required. Two thin Nickel films with the same size were used in this study, separated by an electrical insulating layer. The upper film served as a sensor and the bottom one served as a guard heater. The two Nickel films were operated at a same temperature, so that the Joule heat flux generated by the sensor film transferred to the air with a minimum loss or gain depending on the uncertainties in the film temperature measurements. Analytical solution of the shear stress based on the aforementioned heat flux was obtained. The preliminary results were promising and the estimated wall shear stresses agreed reasonably well with the directly measured values (with errors less than 20%) in a fully developed turbulent pipe flow. The proposed technique can be improved to further increase precisions.

Local wall shear stress measurements with a thin plate submerged in the sublayer in wall turbulent flows

A local wall shear stress measurement technique has been developed using a thin plate, referred to as a sublayer plate which is attached to the wall in the sublayer of a near-wall turbulent flow. The pressure difference between the leading and trailing edges of the plate is correlated to the known wall shear stress obtained in the fully developed turbulent channel flow. The universal calibration curve can be well represented in dimensionless form, and the sensitivity of the proposed method is as high as that of the sublayer fence, even if the sublayer fence is enveloped by the linear sublayer. The results of additional experiments prove that the sublayer plate has fairly good angular resolution in detecting the direction of the local wall shear stress vector.

Evaluation of a film-based wall shear stress measurement technique in a turbulent channel flow

Mean wall shear stress in a turbulent channel flow has been measured using a film-based shear stress sensor with a working principle based on the deformation of a thin elastic film. A direct comparison was made between the elastic film measurements and those using oil-film interferometry in the same experimental facility to evaluate the capability of the novel technique to measure the mean wall shear stress. The results indicate that the film-based sensor measures the wall shear stress within the uncertainty of the oil film measurement for the range of Reynolds numbers considered here, i.e. 2100–2900 based on the friction velocity and the half channel height.

Measurement of surface shear stress vector distribution using shear-sensitive liquid crystal coatings

The global wall shear stress measurement technique using shear-sensitive liquid crystal (SSLC) is extended to wind tunnel measurements. Simple and common everyday equipment is used in the measurement; in particular a tungsten-halogen light bulb provides illumination and a saturation of SSLC coating color change with time is found. Spatial wall shear stress distributions of several typical flows are obtained using this technique, including wall-jet flow, vortex flow generated by a delta wing and junction flow behind a thin cylinder, although the magnitudes are not fully calibrated. The results demonstrate that SSLC technique can be extended to wind tunnel measurements with no complicated facilities used.

Color change characteristics of two shear-sensitive liquid crystal mixtures (BCN/192, BN/R50C) and their application in surface shear stress measurements

A previously determined surface shear stress measurement technique using shear-sensitive liquid crystal (SSLC) coating is extended for use in wind tunnel-like conditions. Simple and common everyday equipment is used in the measurement; in particular a tungsten halogen light bulb provides illumination. The color change characteristics of two SSLC mixtures, BCN192 and BN/R50C, are investigated. BCN/192 is found sensitive to both magnitude and direction of the shear stress at different shear stress levels with little noise, and is suitable for surface shear stress distribution measurements. The spatial shear stress vector distribution beneath a tangential jet is obtained using BCN/192, although the magnitude is not fully calibrated. BN/R50C is found to be sensitive to shear stress magnitude, but only slightly sensitive to shear stress direction at both low and high levels. Moreover, jumps in the reflected hue are found when the viewing orientation is perpendicular to the shear stress direction. These characteristics render the use of BN/R50C for surface shear stress measurement difficult.

Windtunnel studies of surface shear stress vector distribution measurement using shear sensitive liquid crystal coatings

Shear sensitive liquid crystal coating (SSLCC) can measure surface shear stress vector distribution efficiently with a high spatial resolution. The purpose of the present paper is to modify and extend the surface shear stress measurement technique determined by Reda et al. to wind tunnel studies. All the facilities employed in the technique are very common and simple. The measurement technique is applied to surface flow beneath a tangential jet and surface flow in a turbulence wedge generated by a small cylinder in windtunnel flow. Spatially resolved shear stresses over planar surface are obtained, although the magnitude is not fully calibrated.

Infrared Based Wall Shear Stress Measurement Techniques

The paper presented here introduces two novel, infrared based wall shear stress measurement techniques. The first provides wall shear stress visualizations with a high spatial and temporal resolution as well as spatial quantitative information. The other technique enables sensor-based measurements of the wall shear stress magnitude and direction. Both techniques are based on the close link between momentum and heat transport in the boundary layer and correlate the surface temperature distribution of a heated surface (first technique) or a heated spot (second technique), which is measured using infrared thermography, with the wall shear stress. For the spatial qualification and quantification, the temporal surface temperature evolution of a heated structure subjected to a flow is linked to the wall shear stress distribution. The second, sensor-based technique heats a small spot on an otherwise unheated surface. The temperature distribution, or thermal tuft, around this heated spot is closely related to the local wall shear stress magnitude and direction. Results are presented for both techniques and compared to reference measurements and visualizations, respectively. Reference measurements of the wall shear stress were obtained using a skin friction balance; oilflow visualizations were used as a reference visualization technique.

A wall shear stress measurement technique using the thermal wakes of small heated spots

A new thermo-optical method for the measurement of wall shear stresses is presented. The technique exploits that the surface temperature field and the near wall flow are closely linked by correlating the thermal wakes of small heated spots with the wall shear stress. Numerical as well as experimental results are presented and different correlation and design parameters are examined. In contrast to recent works, where the thermal tuft length is used for a correlation with the wall shear stress, other parameters were found to be much better suited for a skin friction calibration. It is also shown that the new method has the unique capability to not only measure the magnitude of the wall shear stress but also its direction.

The wall shear stress produced by the normal impingement of a jet on a flat surface

A method for the theoretical determination of the wall shear stress under impinging jets of various configurations is presented. Axisymmetric and two-dimensional incompressible jets of a wide range of Reynolds numbers and jet heights are considered. Theoretical predictions from this approach are compared with available wall shear stress measurements. These data are critically evaluated based on the method of measurement and its applicability to the boundary layer under consideration. It was found that impingement-region wall shear stress measurements using the electrochemical method in submerged impinging liquid jets provide the greatest accuracy of any indirect method. A unique wall shear stress measurement technique, based on observing the removal of monosized spheres from well-characterized surfaces, was used to confirm the impinging jet analysis presented for gas jets. The technique was also used to determine an empirical relation describing the rise in wall shear stress due to compressibility effects in impinging high-velocity jets.

Comparison of shear stress and wall slip measurement techniques on a linear low density polyethylene

Small gap, torsional, parallel plate experiments allow the measurement of shear stress and slip velocity at shear rates comparable to those measured in slit and capillary rheometers. Using this technique we compare data from parallel plate measurements, where effects of pressure and temperature are negligible, to those from pressure driven flows where they are not. The shear stress for all techniques agrees within experimental error at low shear rates. At higher shear rates the slit and capillary measurements diverge from the parallel plate measurements highlighting the effect of pressure and temperature on viscosity. Also, breakdown of the Cox–Merz rule at slightly higher shear rates is apparent when comparing dynamic and steady shear data from the parallel plate experiments. Since the complex viscosity diverges in the same manner as the viscosity derived from pressure driven flow it seems as if agreement of the complex viscosity with the viscosity derived from pressure driven flow is fortuitous. With the capillary geometry, there is no sign of slip effects at low stress levels probably due to experimental limitations while the torsional rheometer shows slip at all shear stresses. There is no critical stress for the onset of slip but there is a critical stress where slip increases dramatically.

Direct Full Surface Skin Friction Measurement Using Nematic Liquid Crystal Techniques

New techniques for the direct measurement of skin friction using nematic liquid crystal layers are demonstrated. Skin friction measurements can be made using a molecular rotation time technique or an equilibrium orientation technique. A mathematical model describing the molecular dynamics of the nematic liquid crystal layer has been introduced. Theoretical results from the proposed mathematical model are in excellent agreement with the current experimental measurements. It is thus demonstrated that the present model captures the essential physics of the nematic liquid crystal measurement techniques. Estimates based on the variance of the liquid crystal calibration data indicate that skin friction measurements to within ±4 percent should certainly be possible. The techniques offer the considerable advantage of simplicity, without any compromise on the accuracy, relative to other surface shear stress measurement techniques. The full surface measurement capacity of the equilibrium orientation technique is demonstrated by measuring the skin friction distribution around a cylindrical obstruction in a fully developed laminar flow.

On the Accuracy of Wall Similarity Methods in Determining Friction Velocity Over Smooth and Ribletted Surfaces

An accurate determination of friction velocity u^ or, alternatively, of wall shear stress T„ = p u^is of paramount importance in many turbulent boundary layer problems. In applications concerning drag reducing flows, moreover, the wall shear stress is required to be known with high accuracy, since the amount of drag reduction achievable with passive drag reducing techniques is usually of the order of a few percent. A direct evaluation of the gradient dU/dy at the wall can but seldom be achieved because of experimental problems. Integral momentum methods for the evaluation of friction velocity are often too inaccurate and direct shear stress measurement techniques are either inaccurate or expensive. This is why to date the most widely used techniques for determining u^ are indirect, and rely on the well-known law of the wall. Such techniques are known as wall similarity methods. The universality of the law of the wall, which can be deduced from simple dimensional considerations, is, however, subject to many important assumptions and limitations, in particular, the boundary layer must be fully developed and must flow, in equilibrium conditions, over smooth and flat walls, with no longitudinal pressure gradient. Provided these conditions are fully satisfied, the so-called logarithmic region of the mean velocity profile (i.e., that portion where the coordinate normal to the wall is between 30 v/ur and 0.15(5, 6 being the boundary layer thickness) can be expressed in dimensionless form as a function of the friction velocity M (Coles, 1956):